Clock generation apparatus

ABSTRACT

A clock generation apparatus is provided with a frequency phase error calculation circuit  120,  whereby a clock synchronized with burst lock and a line lock clock can be simultaneously generated by a DTO  121  on the basis of frequency information of a DTO  10  and phase error information from a phase comparator  7  and a digital LPF  8.  Therefore, the clock generation apparatus can cope with a system that required plural clocks, and frequency spread is easily carried out by generating spread information by a frequency spread information generation circuit  90,  and adding it in the DTO  121.  As a result, interference to a video terminal from the clock can be reduced, and performance of a video terminal such as a television receiver can be exploited.

FIELD OF THE INVENTION

The present invention relates to a clock generation apparatus forgenerating a clock for operating a signal processing circuit of atelevision receiver and, more particularly, to a clock generationapparatus for generating a clock synchronized with an input videosignal.

BACKGROUND OF THE INVENTION

In recent years, digitization of video signal processing anddiversification of input sources of television receivers areprogressing, and there is employed a clock generation apparatus whichsynchronizes a reference signal such as a horizontal sync signal of avideo signal with a clock for video signal processing.

Hereinafter, a conventional clock generation apparatus will bedescribed.

FIG. 28 is a block diagram illustrating a PLL circuit as a clockgeneration circuit proposed in Japanese Published Patent Application No.Hei. 5-90958. With reference to FIG. 28, reference numeral 301 denotes ahorizontal sync signal input terminal, 302 denotes an analog-to-digitalconverter (ADC), 303 denotes a master clock (MCK) input terminal, 304denotes a digital phase comparator, 305 denotes a frequency divider, 306denotes a digital low-pass filter (LPF), 307 denotes a digital timingoscillator (DTO), 308 denotes a digital-to-analog converter (DAC), 309denotes an analog phase comparator, 310 denotes a reference (REF) signalinput terminal, 311 denotes an analog LPF, 312 denotes an analog VCO,and 313 denotes a clock output terminal.

This PLL circuit is provided with, in addition to a first loop whichcompares the phase of a horizontal sync signal inputted through thehorizontal sync signal input terminal 301 with the phase of a signalbased on an output signal of the DTO 307 by the digital phase comparator304, and controls the DTO 307 with the output of the comparator 304, asecond loop which compares the phase of the output signal of the DTO 307with the phase of an REF signal inputted through the REF signal inputterminal 310 by the analog phase comparator 309, and supplies an outputof the analog VCO 309 to the clock of the DTO 307 according to theoutput of the comparator 309, whereby the PLL circuit is operated as adigital PLL as a whole to generate a clock synchronized with thehorizontal sync signal that is inputted through the horizontal syncsignal input terminal 301.

In the conventional construction, however, the output of the PLL circuitcan be synchronized with only a single sync signal.

Therefore, if the conventional clock generation circuit is used in adevice to which a certain video signal is inputted from among pluralkinds of video signals, like a television receiver in recent years, itis impossible to take a response such as generating a burst lock clockfor a composite signal like a video signal while generating a line lockclock for a component signal.

Further, when receiving a personal computer signal input, it isnecessary to shift the clock phase of the input ADC. However, in theconstruction of the conventional clock generation circuit, it isimpossible to shift the clock-phase of the input ADC when receiving suchpersonal computer signal.

Furthermore, although a video signal and a sync signal are separated inthe personal computer signal, the conventional clock generation circuitcannot respond to that the video signal and the sync signal areseparated.

Moreover, in recent systems, in order to prevent interference due to ahigh frequency to be used, conscious spread of the frequency or the likeis carried out. However, the conventional clock generation circuitcannot take a response such as consciously spreading the frequency.

Furthermore, during a signal processing in a liquid crystal televisionreceiver or the like, it is necessary to generate both a burst clock anda line clock. However, the conventional clock generation circuit cannotgenerate both of these clocks, and therefore, it cannot be used in aliquid crystal television or the like.

SUMMARY OF THE INVENTION

The present invention is made to solve the above-mentioned problems andhas for its object to provide a clock generation apparatus which cangenerate, even when a video signal among plural kinds of signals isinputted, a burst lock clock and a line lock clock suited to each inputsignal without increasing the number of PLLs.

It is another object of the present invention to provide a clockgeneration apparatus which can shift a clock phase of an input ADC whenshifting of the clock phase of the input ADC is required as in apersonal computer signal input, and can respond to that a video signaland a sync signal are separated like in the personal computer input.

It is still another object of the present invention to provide a clockgeneration apparatus which can deal with a case where it is necessary toconsciously diffuse a high frequency to be used by a system to preventinterference from the frequency.

It is a further object of the present invention to provide a clockgeneration apparatus which can generate both of a burst lock clock and aline lock clock simultaneously when both of these sync clocks arerequired.

Other objects and advantages of the invention will become apparent fromthe detailed description that follows. The detailed description andspecific embodiments described are provided only for illustration sincevarious additions and modifications within the scope of the inventionwill be apparent to those of skill in the art from the detaileddescription.

According to a first aspect of the present invention, there is provideda clock generation apparatus comprising: an analog-to-digital converter(hereinafter referred to as an ADC) for converting a video signal onwhich a burst signal or a sync signal is superimposed, which is inputtedthrough a video input terminal, into a digital signal, and outputtingthe digital signal; a burst signal extraction circuit for receiving thevideo signal digitized by the ADC, and extracting the burst signal fromthe video signal; a first phase comparator for comparing the phase ofthe burst signal extracted by the burst signal extraction circuit withthe phase of a clock that operates the ADC; a first digital low-passfilter (hereinafter referred to as a digital LPF) for receiving theoutput of the first phase comparator, and smoothing the output of thefirst phase comparator to output phase error information; a horizontalsync signal separation circuit for receiving the video signal digitizedby the ADC, and separating the horizontal sync signal from the videosignal; a second phase comparator for comparing the phase of thehorizontal sync signal that is separated by the horizontal sync signalseparation circuit with the phase of the clock that operates the ADC; asecond digital LPF for receiving the output of the second phasecomparator, and smoothing the output of the second phase comparator tooutput phase error information; a switch circuit for receiving theoutput of the digital LPF and the output of the second digital LPF, andselecting one of these outputs; a digital timing oscillator (hereinafterreferred to as a DTO) for receiving phase error information that iseither the output of the first digital LPF or the output of the seconddigital LPF, which is outputted from the switch circuit, and outputtinga digital timing signal which is generated at a timing according to thephase error information; a digital-to-analog converter (hereinafterreferred to as a DAC) for converting the digital timing signal outputtedfrom the DTO into an analog signal, and outputting the analog signal;and a PLL for supplying the ADC with a PLL clock according to the analogsignal output from the DAC. Therefore, it is possible to realize a clockgeneration apparatus which can deal with, using a PLL, clock generationin a signal processing for a video signal that requires burst lock, andclock generation in a signal processing for a component signal thatrequires line lock.

According to a second aspect of the present invention, there is provideda clock generation circuit comprising: an ADC for converting a videosignal on which a sync signal is superimposed, which is inputted througha video input terminal, into a digital signal, and outputting thedigital signal; a horizontal sync signal separation circuit forreceiving the video signal digitized by the ADC, and separating thehorizontal sync signal from the video signal; a phase comparator forcomparing the phase of the horizontal sync signal that is separated bythe horizontal sync signal separation circuit with the phase of a clockthat operates the ADC; a digital LPF for receiving the output of thephase comparator, and smoothing the output of the phase comparator tooutput phase error information; a stationary phase error additioncircuit for receiving the phase error information from the digital LPF,and adding stationary phase error to the phase error information; a DTOfor receiving the phase error information outputted from the stationaryphase error addition circuit, and outputting a digital timing signalthat is generated at a timing according to the phase error information;a DAC for converting the digital timing signal outputted from the DTOinto an analog signal, and outputting the analog signal; and a PLL forsupplying the ADC with a PLL clock according to the analog signal outputfrom the DAC. Therefore, it is possible to realize a clock generationapparatus which can shift the phase of a clock with respect to ahorizontal sync signal, and provide a clearer image when sampling apersonal computer signal or the like.

According to a third aspect of the present invention, there is provideda clock generation circuit comprising: a first ADC for converting avideo signal on which a sync signal is superimposed or a sync signal,which is inputted through a first video signal input terminal, into adigital signal, and outputting the digital signal; a second ADC forconverting a video signal inputted through a second video signal inputterminal into a digital signal, and outputting the digital signal; ahorizontal sync signal separation circuit for receiving the video signalor sync signal that is digitized by the first ADC, and separating thehorizontal sync signal from the signal; a phase comparator for comparingthe phase of the horizontal sync signal that is separated by thehorizontal sync signal separation circuit with the phase of a clock thatoperates the first ADC; a digital LPF for receiving the output of thephase comparator, and smoothing the output of the phase comparator tooutput phase error information; a stationary phase error additioncircuit for receiving the phase error information from the digital LPF,and adding stationary phase error to the phase error information; a DTOfor receiving the phase error information outputted from the stationaryphase error addition circuit, and outputting a digital timing signalthat is generated at a timing according to the phase error information;a DAC for converting the digital timing signal outputted from the DTOinto an analog signal, and outputting the analog signal; and a PLL forsupplying the first ADC and the second ADC with a PLL clock according tothe analog signal output from the DAC. Therefore, it is possible torealize a clock generation apparatus which can shift the phase of aclock with respect to a horizontal sync signal, and which can deal witha case where a sync signal and a video signal are separated as in apersonal computer signal, by inputting the sync signal in the first ADCwhile inputting the video signal in the second ADC.

According to a fourth aspect of the present invention, there is provideda clock generation circuit comprising: an ADC for converting a videosignal on which a sync signal is superimposed, which is inputted througha video input terminal, into a digital signal, and outputting thedigital signal; a horizontal sync signal separation circuit forreceiving the video signal digitized by the ADC, and separating thehorizontal sync signal from the video signal; a phase comparator forcomparing the phase of the horizontal sync signal that is separated bythe horizontal sync signal separation circuit with the phase of a clockthat operates the ADC; a stationary phase error addition circuit foradding stationary phase error to the phase error information outputtedfrom the phase comparator; a digital LPF for smoothing the output of thestationary phase error addition circuit; a DTO for receiving the phaseerror information outputted from the digital LPF, and outputting adigital timing signal that is generated at a timing according to thephase error information; a DAC for converting the digital timing signaloutputted from the DTO into an analog signal, and outputting the analogsignal; and a PLL for supplying the ADC with a PLL clock according tothe analog signal output from the DAC. Therefore, it is possible torealize a clock generation apparatus which can shift the phase of aclock with respect to a horizontal sync signal, and which can provide aclearer image when sampling a personal computer signal or the like.

According to a fifth aspect of the present invention, there is provideda clock generation circuit comprising: a first ADC for converting avideo signal on which a sync signal is superimposed or a sync signal,which is inputted through a first video signal input terminal, into adigital signal, and outputting the digital signal; a second ADC forconverting a video signal inputted through a second video signal inputterminal into a digital signal, and outputting the digital signal; ahorizontal sync-signal separation circuit for receiving the video signalor sync signal that is digitized by the first ADC, and separating thehorizontal sync signal from the signal; a phase comparator for comparingthe phase of the horizontal sync signal that is separated by thehorizontal sync signal separation circuit with the phase of a clock thatoperates the first ADC; a stationary phase error addition circuit foradding stationary phase error to the phase error information outputtedfrom the phase comparator; a digital LPF for smoothing the output of thestationary phase error addition circuit; a DTO for receiving the phaseerror information outputted from the digital LPF, and outputting adigital timing signal that is generated at a timing according to thephase error information; a DAC for converting the digital timing signaloutputted from the DTO into an analog signal, and outputting the analogsignal; and a PLL for supplying the first ADC and the second ADC with aPLL clock according to the analog signal output from the DAC. Therefore,it is possible to realize a clock generation apparatus which can shiftthe phase of a clock with respect to a horizontal sync signal, and whichcan deal with a case where a sync signal and a video signal areseparated as in a personal computer signal, by inputting the sync signalin the first ADC while inputting the video signal in the second ADC.

According to a sixth aspect of the present invention, there is provideda clock generation circuit comprising: a first ADC for converting avideo signal on which a sync signal is superimposed or a sync signal,which is inputted through a first video signal input terminal, into adigital signal, and outputting the digital signal; a second ADC forconverting a video signal inputted through a second video signal inputterminal into a digital signal, and outputting the digital signal; ahorizontal sync signal separation circuit for receiving the video signalor sync signal that is digitized by the first ADC, and separating thehorizontal sync signal from the signal; a phase comparator for comparingthe phase of the horizontal sync signal that is separated by thehorizontal sync signal separation circuit with the phase of a clock thatoperates the first ADC; a digital LPF for receiving the output of thephase comparator, and smoothing the output of the phase comparator tooutput phase error information; a DTO for receiving the phase errorinformation outputted from the digital LPF, and outputting a digitaltiming signal that is generated at a timing according to the phase errorinformation; a DAC for converting the digital timing signal outputtedfrom the DTO into an analog signal, and outputting the analog signal; aPLL for supplying the first ADC with a PLL clock according to the analogsignal output from the DAC; a delayed lock loop (hereinafter referred toas a DLL) for receiving the clock outputted from the PLL, and delayingthe clock to output multiple-phase clocks; and a clock selection circuitfor selecting one of the multiple-phase clocks outputted from the DLL,and supplying the second ADC with the selected clock. Therefore, it ispossible to realize a clock generation apparatus which can shift onlythe phase of a clock to be supplied to the second ADC, and which canshift only the phase of sampling of a video signal in a case where async signal and a video signal are separately inputted.

According to a seventh aspect of the present invention, in the clockgeneration apparatus according to the sixth aspect, the DLL comprises aclock input terminal, N stages of variable delay elements for delaying aclock inputted through the clock input terminal, a phase comparator forcomparing the outputs of the variable delay elements with a clockinputted through the clock input terminal, an LPF for smoothing theoutput of the phase comparator, a bias circuit for receiving the outputof the LPF and controlling the delay values of the variable delayelements, and an output terminal for outputting the outputs of the Nstages of variable delay elements.

According to an eighth aspect of the present invention, there isprovided a clock generation circuit comprising: a first ADC forconverting a video signal on which a sync signal is superimposed or async signal, which is inputted through a first video signal inputterminal, into a digital signal, and outputting the digital signal; asecond ADC for converting a video signal inputted through a second videosignal input terminal into a digital signal, and outputting the digitalsignal; a horizontal sync signal separation circuit for receiving thevideo signal or sync signal that is digitized by the first ADC, andseparating the horizontal sync signal from the signal; a phasecomparator for comparing the phase of the horizontal sync signal that isseparated by the horizontal sync signal separation circuit with thephase of a clock that operates the first ADC; a digital LPF forreceiving the output of the phase comparator, and smoothing the outputof the phase comparator to output phase error information; a DTO forreceiving the phase error information outputted from the digital LPF,and outputting a digital timing signal that is generated at a timingaccording to the phase error information; a DAC for converting thedigital timing signal outputted from the DTO into an analog signal, andoutputting the analog signal; a multiple phase output PLL formultiplying the output of the DAC to supply a clock to the first ADC,and simultaneously, outputting an intermediate phase among the multiplephases; and a clock selection circuit for selecting one of the multiplephase clocks outputted from the multiple phase output PLL, and supplyingthe second ADC with the selected clock. Therefore, it is possible torealize a clock generation apparatus which can shift only the phase of aclock to be supplied to the second ADC, and which can shift only thephase of sampling of a video signal in a case where a sync signal and avideo signal are separately inputted.

According to a ninth aspect of the present invention, in the clockgeneration apparatus according to the eighth aspect, the multiple phaseoutput PLL comprises a ring oscillator comprising M stages of variabledelay elements, a frequency divider for converting the frequency of theoutput of the ring oscillator to 1/K, a phase comparator for comparingthe phase of the output from the frequency divider with the phase of anREF signal inputted through an REF signal input terminal, an LPF forreceiving the output of the phase comparator, a bias circuit forreceiving the output of the LPF, and controlling the delay values of thevariable delay elements, and an output terminal for outputting theoutputs of the M stages of variable delay elements.

According to a tenth aspect of the present invention, there is provideda clock generation circuit comprising: an ADC for converting a videosignal on which a sync signal is superimposed, which is inputted througha video input terminal, into a digital signal, and outputting thedigital signal; a horizontal sync signal separation circuit forreceiving the video signal digitized by the ADC, and separating thehorizontal sync signal from the video signal; a phase comparator forcomparing the phase of the horizontal sync signal that is separated bythe horizontal sync signal separation circuit with the phase of a clockthat operates the ADC; a digital LPF for receiving the output of thephase comparator, and smoothing the output of the phase comparator tooutput phase error information; a first DTO for receiving the phaseerror information outputted from the digital LPF, and outputting adigital timing signal that is generated at a timing according to thephase error information; a DAC for converting the digital timing signaloutputted from the first DTO into an analog signal, and outputting theanalog signal; a first PLL for supplying the first ADC with a PLL clockaccording to the analog signal output from the DAC; a frequency spreadinformation generation circuit for generating frequency spreadinformation; a second DTO for receiving the phase error informationoutputted from the digital LPF and the frequency spread informationgenerated by the frequency spread information generation circuit, andoutputting a digital timing signal; a second DAC for converting thedigital timing signal outputted from the second DTO into an analogsignal; and a PLL for outputting a PLL clock according to the analogsignal output from the second DAC. Therefore, it is possible to realizea clock generation apparatus which can diffuse the frequency of a clockto reduce interference that occurs in the apparatus.

According to an eleventh aspect of the present invention, in the clockgeneration apparatus according to the tenth aspect, the frequency spreadinformation generation circuit comprises a timing generation circuitwhich has a clock input terminal and counts the clocks inputted throughthe clock input terminal to generate a timing, and an up/down counterwhich receives a timing waveform generated by the timing generationcircuit and switches between up counting and down counting.

According to a twelfth aspect of the present invention, there isprovided a clock generation circuit comprising: an ADC for converting avideo signal on which a sync signal is superimposed, which is inputtedthrough a video input terminal, into a digital signal, and outputtingthe digital signal; a horizontal sync signal separation circuit forreceiving the video signal digitized by the ADC, and separating thehorizontal sync signal from the video signal; a phase comparator forcomparing the phase of the horizontal sync signal that is separated bythe horizontal sync signal separation circuit with the phase of a clockthat operates the ADC; a digital LPF for receiving the output of thephase comparator, and smoothing the output of the phase comparator tooutput phase error information; a DTO for receiving the phase errorinformation outputted from the digital LPF, and outputting a digitaltiming signal that is generated at a timing according to the phase errorinformation; a DAC for converting the digital timing signal outputtedfrom the DTO into an analog signal, and outputting the analog signal;and a first PLL for supplying the first ADC with a PLL clock accordingto the analog signal output from the DAC; a second PLL for receiving theoutput of the DAC; a frequency spread information generation circuit forgenerating frequency spread information; a PWM circuit for convertingthe output of the frequency spread information generation circuit intoan analog value; an analog LPF for receiving the output of the PWMcircuit; and a resistor for adding the output of the analog LPF to acontrol voltage of a VCO which is a component of the second PLL.Therefore, it is possible to realize a clock generation apparatus whichcan diffuse the frequency of a clock to reduce interference that occursin the apparatus.

According to a thirteenth aspect of the present invention, in the clockgeneration apparatus according to the twelfth aspect, the frequencyspread information generation circuit comprises a timing generationcircuit which has a clock input terminal and counts the clocks inputtedthrough the clock input terminal to generate a timing, and an up/downcounter which receives a timing waveform generated by the timinggeneration circuit and switches between up counting and down counting.

According to a fourteenth aspect of the present invention, there isprovided a clock generation apparatus comprising: an ADC for convertinga video signal on which a burst signal or a sync signal is superimposed,which is inputted through a video input terminal, into a digital signal,and outputting the digital signal; a burst signal extraction circuit forreceiving the video signal digitized by the ADC, and extracting theburst signal from the video signal; a first phase comparator forcomparing the phase of the burst signal extracted by the burst signalextraction circuit with the phase of a clock that operates the ADC; afirst digital LPF for receiving the output of the first phasecomparator, and smoothing the output of the first phase comparator tooutput phase error information; a first DTO for receiving the phaseerror information outputted from the first digital LPF, and outputting adigital timing signal that is generated at a timing according to thephase error information; a first DAC for converting the digital timingsignal outputted from the first DTO into an analog signal, andoutputting the analog signal; a first PLL for supplying the ADC with aPLL clock according to the analog signal output from the first DAC; ahorizontal sync signal separation circuit for receiving the video signaldigitized by the ADC, and separating a horizontal sync signal from thevideo signal; a second phase comparator for comparing the phase of thehorizontal sync signal separated by the horizontal sync signalseparation circuit with the phase of the clock that operates the ADC; asecond digital LPF for receiving the output of the second phasecomparator, and smoothing the output of the second phase comparator tooutput phase error information; a frequency phase calculation circuitfor receiving frequency information and phase information of the outputof the first DTO and, the phase error information outputted from thesecond digital LPF, and outputting frequency information and phaseinformation which are needed to generate a clock synchronized with thehorizontal sync signal; a second DTO for receiving the output of thefrequency phase calculation circuit, and outputting a digital timingsignal which is generated at a timing according to the output of thefrequency phase calculation circuit; a second DAC for converting thedigital timing signal outputted from the second DTO into an analogsignal, and outputting the analog signal; and a second PLL foroutputting a PLL clock according to an analog signal output from thesecond DAC. Therefore, it is possible to realize a clock generationapparatus which can simultaneously generate a clock synchronized with aburst signal and a clock synchronized with a horizontal sync signal,which are required in a system such as a liquid crystal television orthe like.

According to a fifteenth aspect of the present invention, in the clockgeneration apparatus according to the fourteenth aspect, the frequencyphase calculation circuit comprises a line lock clock frequencyinformation generation circuit for receiving phase error informationbetween a burst lock clock and the horizontal sync signal, which is anoutput of the second digital LPF, and frequency information of a burstlock clock of the first DTO; and a line lock clock phase informationgeneration circuit for receiving frequency information of a line lockclock outputted from the line lock clock frequency informationgeneration circuit, phase information of the burst lock clock of thefirst DTO, the frequency information of the burst lock clock, and thephase error information between the burst lock clock and the horizontalsync signal. Therefore, frequency information and phase informationwhich are required for line lock can be calculated with only digitaldata, whereby a stable system can be set up.

According to a sixteenth aspect of the present invention, in the clockgeneration apparatus according to the fifteenth aspect, the line lockclock frequency information generation circuit comprises a frequencyratio calculation circuit which receives the frequency division ratio ofthe burst lock clock to the horizontal sync signal during standardoperation, the frequency division ratio of the line lock clock to thehorizontal sync signal, and a phase difference between the actualhorizontal sync signal and the signal obtained by frequency dividing theclock during burst lock clock standard operation, and calculates theratio of frequencies between the burst lock clock and the line lockclock; and a multiplier which multiplies the frequency information ofthe burst lock clock by the ratio of the frequencies that is calculatedby the frequency ratio calculation circuit to output frequencyinformation of the line lock clock. Therefore, frequency information canbe generated by ratio calculation and multiplication using a factorobtained by the ratio calculation, whereby the clock generationapparatus can easily be incorporated in a digital circuit.

According to a seventeenth aspect of the present invention, in the clockgeneration apparatus according to the fifteenth aspect, the line lockphase information generation circuit comprises a timing generationcircuit for generating a timing for setting an initial value of thesecond DTO that generates the line lock clock, and a timing forcapturing the phase information of the burst lock clock; a linelock/burst lock frequency ratio calculation circuit for calculating theratio of frequencies from the burst lock frequency information and theline lock frequency information; and an initial value calculationcircuit for calculating an initial value of the second DTO thatgenerates the line lock clock from the phase difference between theactual horizontal sync signal and the signal obtained by frequencydividing the clock during burst lock clock standard operation, the phaseinformation of the burst lock clock, and the frequency ratio obtained bythe line lock/burst lock frequency ratio calculation circuit. Therefore,frequency information of the DTO and phase information of the DTO forburst lock and line lock can be obtained by four arithmetic operations,whereby the clock generation apparatus can be incorporated in a digitalcircuit.

According to an eighteenth aspect of the present invention, there isprovided a clock generation apparatus comprising: an ADC for convertinga video signal on which a burst signal or a sync signal is superimposed,which is inputted through a video input terminal, into a digital signal,and outputting the digital signal; a burst signal extraction circuit forreceiving the video signal digitized by the ADC, and extracting theburst signal from the video signal; a first phase comparator forcomparing the phase of the burst signal extracted by the burst signalextraction circuit with the phase of a clock that operates the ADC; afirst digital LPF for receiving the output of the first phasecomparator, and smoothing the output of the first phase comparator tooutput phase error information; a first DTO for receiving the phaseerror information outputted from the first digital LPF, and outputting adigital timing signal that is generated at a timing according to thephase error information; a first DAC for converting the digital timingsignal outputted from the first DTO into an analog signal, andoutputting the analog signal; a first PLL for supplying the ADC with aPLL clock according to the analog signal output from the first DAC; ahorizontal sync signal separation circuit for receiving the video signaldigitized by the ADC, and separating a horizontal sync signal from thevideo signal; a second phase comparator for comparing the phase of thehorizontal sync signal separated by the horizontal sync signalseparation circuit with the phase of the clock that operates the ADC; asecond digital LPF for receiving the output of the second phasecomparator, and smoothing the output of the second phase comparator tooutput phase error information; a frequency spread informationgeneration circuit for generating frequency spread information; afrequency phase calculation circuit for receiving frequency informationand phase information of the output of the first DTO, and the phaseerror information outputted from the second-digital LPF, and outputtingfrequency information and phase information which are needed to generatea clock synchronized with the horizontal sync signal; a second DTO forreceiving the output of the frequency phase calculation circuit and theoutput of the frequency spread information generation circuit, andoutputting a digital timing signal; a second DAC for converting thedigital timing signal outputted from the second DTO into an analogsignal, and outputting the analog signal; and a second PLL foroutputting a PLL clock according to an analog signal output from thesecond DAC. Therefore, it is possible to realize a clock generationapparatus which can generate a line lock clock as well as a burst lockclock, and which can diffuse the frequencies of the clocks to reduceinterference that occurs in the apparatus.

According to a nineteenth aspect of the present invention, in the clockgeneration apparatus according to the eighteenth aspect, the frequencyspread information generation circuit comprises a timing generationcircuit which has a clock input terminal and counts the clocks inputtedthrough the clock input terminal to generate a timing, and an up/downcounter which receives a timing waveform generated by the timinggeneration circuit and switches between up counting and down counting.

According to a twelfth aspect of the present invention, there isprovided a clock generation apparatus comprising: an ADC for convertinga video signal on which a burst signal or a sync signal is superimposed,which is inputted through a video input terminal, into a digital signal,and outputting the digital signal; a burst signal extraction circuit forreceiving the video signal digitized by the ADC, and extracting theburst signal from the video signal; a first phase comparator forcomparing the phase of the burst signal extracted by the burst signalextraction circuit with the phase of a clock that operates the ADC; afirst digital LPF for receiving the output of the first phasecomparator, and smoothing the output of the first phase comparator tooutput phase error information; a first DTO for receiving the phaseerror information outputted from the first digital LPF, and outputting adigital timing signal that is generated at a timing according to thephase error information; a first DAC for converting the digital timingsignal outputted from the first DTO into an analog signal, andoutputting the analog signal; a first PLL for supplying the ADC with aPLL clock according to the analog signal output from the first DAC; ahorizontal sync signal separation circuit for receiving the video signaldigitized by the ADC, and separating a horizontal sync signal from thevideo signal; a second phase comparator for comparing the phase of thehorizontal sync signal separated by the horizontal sync signalseparation circuit with the phase of the clock that operates the ADC; asecond digital LPF for receiving the output of the second phasecomparator, and smoothing the output of the second phase comparator tooutput phase error information; a frequency phase calculation circuitfor receiving frequency information and phase information of the outputof the first DTO, and the phase error information outputted from thesecond digital LPF, and outputting frequency information and phaseinformation which are needed to generate a clock synchronized with thehorizontal sync signal; a second DTO for receiving the output of thefrequency phase calculation circuit, and outputting a digital timingsignal that is generated at a timing according to the output of thefrequency phase calculation circuit; a second DAC for converting thedigital timing signal outputted from the second DTO into an analogsignal, and outputting the analog signal; a second PLL for receiving theoutput of the second DAC; a frequency spread informationgeneration-circuit for generating frequency spread information; a PWMcircuit for converting the output of the frequency spread informationgeneration circuit into an analog value; an analog LPF for receiving theoutput of the PWM circuit; and a resistor for adding the output of theanalog LPF to a control voltage of the VCO which is a component of thesecond PLL. Therefore, it is possible to realize a clock generationapparatus which can generate a line lock clock as well as a burst lockclock, and which can diffuse the frequencies of the clocks to reduceinterference that occurs in the apparatus.

According to a twenty-first aspect of the present invention, in theclock generation apparatus according to the twentieth aspect, thefrequency spread information generation circuit comprises a timinggeneration circuit which has a clock input terminal and counts theclocks inputted through the clock input terminal to generate a timing,and an up/down counter which receives a timing waveform generated by thetiming generation circuit and switches between up counting and downcounting.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a block diagram illustrating the construction of a clockgeneration apparatus according to a first embodiment of the presentinvention.

FIG. 2 is a block diagram illustrating the construction of a DTO 10shown in FIG. 1.

FIG. 3 is a block diagram illustrating the constructions of a horizontalsync signal separation circuit 6 and a digital phase comparator 7 shownin FIG. 1.

FIG. 4 is a timing chart for explaining the operations of the horizontalsync signal separation circuit 6 and the digital phase comparator 7shown in FIG. 1.

FIG. 5 is a block diagram illustrating the construction of a clockgeneration apparatus according to a second embodiment of the presentinvention.

FIG. 6 is a timing chart for explaining a video signal samplingoperation.

FIG. 7 is a diagram illustrating examples of a horizontal sync signalseparation circuit 6, a phase comparator 7, and a stationary phase erroraddition circuit 45 shown in FIG. 5.

FIG. 8 is a diagram illustrating other examples of a horizontal syncsignal separation circuit 6, a phase comparator 7, and a stationaryphase error addition circuit 45 shown in FIG. 5.

FIG. 9 is a block diagram illustrating the construction of a clockgeneration apparatus according to a third embodiment of the presentinvention.

FIG. 10 is a block diagram illustrating the construction of a clockgeneration apparatus according to a fourth embodiment of the presentinvention.

FIG. 11 is a block diagram illustrating the construction of a DLL 50shown in FIG. 10.

FIG. 12 is a timing waveform diagram for explaining the operation of theDLL 50 shown in FIG. 10.

FIG. 13 is a block diagram illustrating the construction of a clockgeneration apparatus according to a fifth embodiment of the presentinvention.

FIG. 14 is a block diagram illustrating the construction of amultiple-phase output analog PLL 65 shown in FIG. 13.

FIG. 15 is a block diagram illustrating the construction of a clockgeneration apparatus according to a sixth embodiment of the presentinvention.

FIG. 16 is a block diagram illustrating the construction of a DTO 91shown in FIG. 15.

FIG. 17 is a block diagram illustrating the construction of a frequencyspread information generation circuit 90 shown in FIG. 15.

FIG. 18 is a timing chart for explaining the operation of the frequencyspread information generation circuit 90.

FIG. 19 is a block diagram illustrating the construction of a clockgeneration apparatus according to a seventh embodiment of the presentinvention.

FIG. 20 is a block diagram illustrating the construction of an analogPLL 103 shown in FIG. 19.

FIG. 21 is a block diagram illustrating the construction of a clockgeneration apparatus according to an eighth embodiment of the presentinvention.

FIG. 22 is a simple block diagram illustrating the constructions of afrequency phase calculation circuit 120 and a DTO 121 shown in FIG. 21.

FIG. 23 is a block diagram specifically illustrating the constructionsof the frequency phase calculation circuit 120 and the DTO 121 includinga DTO 10.

FIG. 24 is a waveform diagram for explaining the conception of afrequency calculation method in the clock generation apparatus accordingto the eighth embodiment.

FIG. 25 is a waveform diagram for explaining a phase informationcalculation method in the clock generation apparatus according to theeighth embodiment.

FIG. 26 is a block diagram illustrating the construction of a blockdiagram illustrating the construction of a clock generation apparatusaccording to a ninth embodiment of the present invention.

FIG. 27 is a block diagram illustrating the construction of a clockgeneration apparatus according to a tenth embodiment of the presentinvention.

FIG. 28 is a block diagram illustrating the conventional clockgeneration apparatus.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Hereinafter, clock generation apparatuses according to preferredembodiments of the present invention will be described with reference tothe drawings.

Embodiment 1

FIG. 1 is a block diagram illustrating the construction of a clockgeneration circuit according to a first embodiment of the presentinvention.

In FIG. 1, reference numeral 1 denotes a video signal input terminal, 2denotes an analog-to-digital converter (ADC), 3 denotes a burstextraction circuit, 4 denotes a digital phase comparator, 5 denotes adigital LPF, 6 denotes a horizontal sync signal separation circuit, 7denotes a digital phase comparator, 8 denotes a digital LPF, 9 denotes aswitch, 10 denotes a digital timing oscillator (DTO), 11 denotes adigital-to-analog converter (DAC), 12 denotes an analog PLL, 13 denotesa master clock (MCK) input terminal, 14 denotes a clock output terminal,and 21 denotes a center frequency information input terminal.

FIG. 2 is a block diagram illustrating the construction of the DTO 10shown in FIG. 1, FIG. 3 is a block diagram illustrating theconstructions of the horizontal sync signal separation circuit 6 and thedigital phase comparator 7 shown in FIG. 1, and FIG. 4 is a waveformdiagram of the horizontal sync signal separation circuit shown in FIG.3. In FIG. 2, reference numeral 20 denotes a phase error informationinput terminal, 21 denotes a center frequency information inputterminal, 22 and 26 denote adders, 23 denotes a flip-flop, 24 denotes aSIN-ROM, and 25 denotes a waveform output terminal. In FIG. 3, referencenumeral 30 denotes an input terminal for a video signal, or a horizontalsync signal, or a sync signal, 31 denotes a clock input terminal, 32denotes a slicer, 33 denotes a frequency division circuit, 34 denotes adata addition/subtraction unit, 35 denotes a sync signal level detector,36 denotes a divider, and 37 denotes a phase error information inputterminal.

Hereinafter, the operation of the clock generation circuit according tothe first embodiment will be described with reference to the drawings.

A composite signal represented by a video signal or a component signalas a signal from a DVD is inputted through the video signal inputterminal 1. The input video signal is analog-to-digital converted by theADC 2. The signal digitized by the ADC 2 should be subjected to videosignal processing with a burst lock clock if the input signal is acomposite signal. When the input signal is a component signal, it shouldbe subjected to video signal processing with a line lock clock. In thisfirst embodiment, when the input signal is a composite signal, a burstsignal superimposed on the composite signal is extracted by the burstsignal extraction circuit 3, and the phase of the extracted burst signalis compared with the phase of the clock by the digital phase comparator4, and then the result is smoothed by the digital LPF 5, therebyoutputting phase error information between the burst signal and theclock. The switch 9 is controlled by, for example, a control signalaccording to switching of the video signal inputted to the video signalinput terminal 1. When the input signal is a composite signal, theswitch 9 is controlled so as to pass the output of the digital LPF 5between the output of the digital LPF 5 and the output of the digitalLPF 8. Thereby, the output of the digital LPF 5 is input to the DTO 10,and a sinusoidal wave according to the phase error information isoutputted from the DTO 10 and the DAC 11, and then the sinusoidal waveis multiplied by the analog PLL 12, whereby a clock required in thesystem is outputted from the clock output terminal 14. Simultaneously,the clock is fed back to the ADC 2 as a sampling clock, whereby a clocksynchronized with the burst signal superimposed on the composite signalcan be generated. The clock to be inputted to the MCK clock inputterminal 13 should be a stable clock having a frequency higher than thatof the burst signal.

On the other hand, in the video signal processing for the input signalwhich is a component signal, it is necessary to generate a line lockclock synchronized with the horizontal sync signal. In this case, theinput signal is digitized by the ADC 2, and thereafter, only thehorizontal sync signal is separated by the horizontal sync signalseparation circuit 6. Then, the phase of the horizontal sync signal iscompared with the phase of the clock by the digital phase comparator 7,and smoothed by the digital LPF 8, thereby outputting phase errorinformation between the horizontal sync signal and the clock. When theinput signal is a component-signal, the switch 9 is controlled so as topass the output of the digital LPF 8, between the output of the digitalLPF 5 and the output of the digital LPF 8. Thereby, the output of thedigital LPF 8 is input to the DTO 10, and the DTO 10 and the DAC 11output a sinusoidal wave according to the phase error information, andthe sinusoidal wave is multiplied by the analog PLL 12, whereby a clockrequired in the system is outputted from the clock output terminal 14.Simultaneously, the clock is fed back to the ADC 2 as a sampling clock,thereby generating a clock synchronized with the horizontal sync signal.

The DTO 10 will be described with reference to FIG. 2. The DTO 10receives either the phase error information generated in the LPF 5 orthe phase error information generated in the LPF 8, which is selected bythe switch 9, through the phase error information input terminal 20, andfurther, it receives center frequency information of a frequency to begenerated essentially, through the center frequency information inputterminal 21. Since the center frequency information depends on thefrequency that is needed in the system, it may be a fixed value. Thesetwo pieces of information are added by the adder 26, and thereafter,accumulated by the adder 22 and the flip-flop 23, thereby outputting asawtooth wave of a desired frequency as an output of the flip-flop 23.The output of the flip-flop 23 is input to the SIN-ROM 24 whereinsinusoidal waveform information is written, and decoded to be output asa sinusoidal wave from the sinusoidal wave output terminal 25, wherebythe DTO oscillates the sinusoidal wave.

Hereinafter, the operations of the horizontal sync signal separationcircuit 6 and the digital phase comparator 7 will be described withreference to FIGS. 3 and 4. A digitized video signal and a sync signalare input through the signal input terminal 30. A video signal A shownin FIG. 4 is an enlarged portion of the horizontal sync signal that issuperimposed on the video signal or the sync signal. These signalsinputted through the signal input terminal 30 are sliced at anappropriate level (level a in FIG. 4) by the slicer 32, and thehorizontal sync signal is extracted. The phase of the extracted signalis compared with the phase of a clock that is obtained by 1/N frequencydividing the clock (the clock outputted from the analog PLL 12) inputtedthrough the clock input terminal 31 by the frequency divider 33. Afrequency-divided output B shown in FIG. 4 is an enlarged view of aphase comparison part of the frequency-divided output B. The processingof phase comparison is as follows. When the frequency-divided output Bis a logic value “0” in the data addition/subtraction unit 34, theamplitude of the sync signal is added. In FIG. 4, c which is adifference between the slice level a and the sync signal level b isadded. When the frequency-divided output B is a logic value “1”, theamplitude of the sync signal is subtracted. In FIG. 4, c which is adifference between the slice level a and the sync signal level b issubtracted. As a result, a data addition/subtraction unit output C isobtained, and a resultant d is a phase difference. In this circuit, amaximum value of c that is a difference of the level b of the syncsignal is detected by the sync signal level detector 35 so that theweight of the phase error information is not changed depending on thelevel of the signal amplitude, and the d that is the final output of thedata addition/subtraction unit is divided by this value to obtain phaseerror information from the phase error information output terminal 37.

As described above, the first embodiment of the present invention isprovided with the first phase comparator 4 for comparing the phase ofthe burst signal extracted from the video signal that is digitized bythe ADC 2 with the phase of the clock for operating the ADC 2; the firstdigital LPF 5 for smoothing the output of the first phase comparator 4to output phase error information; the second phase comparator 7 forcomparing the phase of the horizontal sync signal that is separated fromthe video signal digitized by the ADC 2 with the phase of the clock foroperating the ADC 2; the second digital LPF 8 for smoothing the outputof the second phase comparator 7 to output phase error information; andthe switching circuit 9 which receives the output of the first digitalLPF 5 and the output of the second digital LPF 8, thereby realizing aclock generation apparatus which can deal with, using a single PLL,clock generation during signal processing for a video signal thatrequires burst lock, and clock generation during signal processing for acomponent signal that requires line lock. Although particularly a DAC oran analog PLL is a circuit that occupies a large area in a miniaturizedsemiconductor device or the like, in this first embodiment the DAC andthe analog PLL can share a common circuit, resulting in a reduction inthe scale of the semiconductor device.

Embodiment 2

FIG. 5 is a block diagram illustrating the construction of a clockgeneration apparatus according to a second embodiment of the presentinvention. In FIG. 5, reference numeral 40 denotes a signal inputterminal, 2 denotes an ADC, 6 denotes a horizontal sync signalseparation circuit, 7 denotes a phase comparator, 45 denotes astationary phase error addition circuit, 8 denotes a digital LPF, 10denotes a DTO, 11 denotes a DAC, 12 denotes an analog PLL, 13 denotes aMCK input terminal, 14 denotes a clock output terminal, 42 denotes avideo signal input terminal, and 43 denotes an ADC. Further, FIG. 6 is adiagram illustrating waveforms of video sampling clocks for the ADC 2and the ADC 43.

Hereinafter, the operation of the clock generation apparatus accordingto the second embodiment will be described with reference to thedrawings. A video signal on which a horizontal sync signal issuperimposed is input through the signal input terminal 40. The inputvideo signal is processed in like manner as described for the firstembodiment by the ADC 2, the horizontal sync signal separation circuit6, and the phase comparator 7, and a difference in phases between thehorizontal sync signal and the clock is outputted as phase errorinformation. Then, stationary phase error information is added to thephase error information by the stationary phase error addition circuit45, the phase error information is smoothed by the digital LPF 8, aclock is generated using the DTO 10, the DAC 11, and the analog PLL 12,and the clock is fed back to the ADC 2 as a sampling clock, therebygenerating a clock synchronized with the horizontal sync signal whileholding the stationary phase error. Since the clock generation apparatusaccording to the second embodiment is provided with the stationary phaseerror addition circuit 45, the ADC can be sampled with a clock whosephase is shifted while synchronizing the clock with the horizontal syncsignal. During sampling of a video signal that is outputted from the DACwhile being synchronized with the clock, like a video signal from apersonal computer, when the video signal from the DAC has a waveform asshown by a video signal D in FIG. 6, if the timing of the clock thatperforms sampling is the timing of a sampling clock E, the data in themiddle of change are undesirably captured, and thereby a clear imagecannot be obtained. However, a clear image can be obtained by shiftingthe phase, and performing sampling at the timing of a sampling clock F.Further, it is possible to cope with a personal computer signal in whicha video signal and a horizontal sync signal are separated, by providingthe ADC 43, inputting the sync signal through a signal input terminal 40and the video signal through a video signal input terminal 42, andsampling the video signal with the second ADC 43.

FIG. 7 is a block diagram illustrating the constructions of thehorizontal sync signal separation circuit 6, the phase comparator 7, andthe stationary phase error addition circuit 45.

In FIG. 7, reference numeral 30 denotes a signal input terminal, 31denotes a clock input terminal, 32 denotes a slicer, 33 denotes afrequency divider, 34 denotes a data addition/subtraction unit, 35denotes a sync signal level detector, 36 denotes a divider, and 37denotes a phase error information output terminal. These constituentsare identical to those of the horizontal sync signal separation circuit6 and the phase comparator 7 of the clock generation apparatus accordingto the first embodiment. Further, 46 denotes an adder, and 47 denotes astationary phase error information input terminal. The operations of thesignal input terminal 30, the clock input terminal 31, the slicer 32,the divider 33, the data addition/subtraction unit 34, the sync signallevel detector 35, and the divider 36 are identical to the operations ofthe corresponding parts of the clock generation apparatus according tothe first embodiment. Stationary phase error information is inputtedthrough the stationary phase error information input terminal 47, andphase error information outputted from the divider 36 is added to thestationary phase error information by the adder 46, thereby outputtingthe phase error information including the stationary phase error fromthe phase error information output terminal 37. Further, since the phaseerror information including the stationary phase error can be generatedby adding the arbitrary stationary phase error information by the adder46, the clock generation apparatus can be easily implemented by adigital circuit and incorporated in an LSI or the like.

As described above, since the clock generation apparatus according tothe second embodiment is provided with the stationary phase erroraddition circuit 45 for adding the stationary phase error to the phaseerror information, the phase of the clock can be shifted with respect tothe horizontal sync signal, thereby realizing a clock generationapparatus which can provide a clearer image when sampling a personalcomputer signal or the like.

While in FIG. 7 the adder 46 for adding the stationary phase error isprovided in the stage after the divider 36, it may be provided in thestage before the divider 36 as shown in FIG. 8 with the same effects asmentioned above.

Embodiment 3

FIG. 9 is a block diagram illustrating the construction of a clockgeneration apparatus according to a third embodiment of the presentinvention. In FIG. 9, the same reference numerals as those shown in FIG.5 denote the same or corresponding parts, and reference numeral 41denotes a stationary phase error addition circuit.

Hereinafter, the operation of the clock generation apparatus accordingto the third embodiment will be described. A video signal on which ahorizontal sync signal is superimposed is inputted through the signalinput terminal 40. The inputted video signal is processed in the sameway as described for the first embodiment by the ADC 2, the horizontalsync signal separation circuit 6, and the phase comparator 7, whereby aphase difference between the horizontal sync signal and the clock isoutputted as phase error information. This phase error information issmoothed by the digital LPF 8, and thereafter, stationary phase errorinformation is added to the phase error information by the stationaryphase error addition circuit 41. Then, a clock is generated using theDTO 10, the DAC 11, and the analog PLL 12, and this clock is fed back tothe ADC 2 as a sampling clock, thereby generating a clock synchronizedwith the horizontal sync signal while holding the stationary phaseerror.

In contrast to the clock generation apparatus according to the secondembodiment wherein the stationary phase error addition circuit isprovided in the stage before the digital LPF 8, the clock generationapparatus according to the third embodiment has the stationary phaseerror addition circuit 41 in the stage after the digital LPF 8. Also inthis construction, the same effects as described for the secondembodiment can be achieved.

Embodiment 4

FIG. 10 is a block diagram illustrating the construction of a clockgeneration apparatus according to a fourth embodiment of the presentinvention. In FIG. 10, the same reference numerals as those shown inFIG. 5 denote the same or corresponding parts, and reference numeral 50denotes a delayed lock loop (DLL), 15 and 16 denote clock outputterminals, and 17 denotes a control input terminal.

Hereinafter, the operation of the clock generation apparatus accordingto the fourth embodiment will be described with reference to thedrawings. The operation of the circuit comprising the ADC 2, thehorizontal sync signal separation circuit 6, the phase comparator 7, thedigital LPF 8, the DTO 10, the DAC 11, the analog PLL 12, the MCK inputterminal 13, the signal input terminal 40, the video signal inputterminal 42, and the ADC 43 is identical to the operation of thecorresponding part of the clock generation apparatus according to thesecond embodiment. A clock synchronized with the horizontal sync signalcan be generated by outputting a clock generated in the analog PLL 12from the clock output terminal 15 and, simultaneously, feeding back theclock to the ADC 2 as a sampling clock. Further, in the clock generationapparatus according to the fourth embodiment, multiple-phase clocks aregenerated by the DLL 50 from the clock generated in the analog PLL 12.FIG. 11 is a block diagram illustrating an example of construction ofthe DLL 50 in the clock generation apparatus according the fourthembodiment. In FIG. 11, reference numeral 51 denotes a clock inputterminal, 52 to 55 denote delay elements, 56 denotes a phase comparator,57 denotes an LPF, and 58 denotes a BIAS circuit. In FIG. 11, there arefour stages of delay elements. FIG. 12 is a diagram illustratingwaveforms of outputs from the DLL 50. When a clock generated in theanalog PLL 12 is inputted through the clock input terminal 51, theinputted clock is delayed by the delay elements 52 to 55, and then, inthe phase comparator 56, the phase of the delayed clock is compared withthe phase of a clock that is one clock behind the clock inputted throughthe clock input terminal 51. The result of phase comparison is smoothedby the LPF 58. The smoothed value is inputted to the BAIS circuit,wherein a voltage that varies the delay values of the delay elements 52to 55 is generated. By varying the delay values of the delay elements 52to 55, clocks that are delayed by ¼ from each other can be generated tobe outputted from the multiple-phase clock output terminals 59 to 62.Then, the clock selection circuit 63 which operates on the basis of thecontrol input supplied from the control input terminal 17 selects one ofthe multiple-phase clocks which are generated by the DLL 50, and thephase of the clock is shifted with respect to the clock generated in theanalog PLL 12 to be outputted from the clock output terminal 16, andsimultaneously, this clock is used as a sampling clock for the ADC 43.

As described above, the clock generation apparatus according to thefourth embodiment is provided with the DLL 50 to generate themultiple-phase clocks, and one of the clocks is selected by the clockselection circuit 63 to shift the phase of the clock. Therefore, likethe second embodiment, when sampling a personal computer signal or thelike, a clearer image can be obtained, and further, only the phase ofthe sampling clock of the ADC 43, i.e., the clock that samples the videosignal, can be shifted, whereby disordering at phase change can besuppressed.

Embodiment 5

FIG. 13 is a block diagram illustrating the construction of a clockgeneration apparatus according to a fifth embodiment of the presentinvention. In FIG. 13, the same reference numerals as those shown inFIG. 10 denote the same or corresponding parts, and reference numeral 64denotes a clock selection circuit, and 65 denotes a multiple-phaseoutput analog PLL.

Hereinafter, the operation of the clock generation apparatus accordingto the fifth embodiment will be described with reference to thedrawings. The operation of the circuit comprising the ADC 2, thehorizontal sync signal separation circuit 6, the phase comparator 7, thedigital LPF 8, the DTO 10, the DAC 11, the MCK input terminal 13, thesignal input terminal 40, the video signal input terminal 42, and theADC 43 is identical to the operation of the corresponding part of theclock generation apparatus according to the second embodiment. The clockgeneration apparatus according to the fifth embodiment employs themultiple-phase output analog PLL 65 as an analog PLL for multiplying asinusoidal wave generated by the DTO 10 and DAC 11, and outputs, as amultiple-phase clock, a clock having an intermediate phase of a clockthat can be generated in the PLL.

FIG. 14 is a block diagram illustrating the construction of themultiple-phase output analog PLL 65 in the clock generation apparatusaccording to the fifth embodiment. In FIG. 14, reference numerals 70 to74 denote inversion delay elements, 75 denotes a frequency divisioncircuit, 76 denotes a phase comparator, 77 denotes an LPF, 78 denotes abias circuit, 79 denotes an REF signal input terminal, and 80 to 84denote multiple-phase clock output terminals. In the multiple-phaseoutput analog PLL 65, a clock is generated with a ring oscillator usingthe inversion delay elements 70 to 74, the generated clock isfrequency-divided by the frequency division circuit 75, the phase of theclock is compared with the phase of the REF signal that is inputtedthrough the REF signal input terminal by the phase comparator 76, andthe result of phase comparison is smoothed by the LPF 77. The smoothedsignal is converted into a voltage that controls the delay values of theinversion delay elements 70 to 74 in the BAIS circuit 78. It is possibleto generate a clock that is phase-synchronized with the REF signal (theoutput signal of the DAC 11) and is multiplied, by controlling the delayvalues of the inversion delay elements 70 to 74 with the controlvoltage, a clock, and this clock signal is outputted from the clockoutput terminal 15, and simultaneously, is fed back to the ADC 2 as asampling clock, thereby generating a clock synchronized with thehorizontal sync signal. At this time, the outputs of the inversion delayelements 70 to 74 are outputted to the multiple-phase clock outputterminals 80 to 84, respectively, whereby a clock of an intermediatephase of the multiple phases can be outputted. Then, the clock selectioncircuit 64 that is operated on the basis of the control input suppliedfrom the control input terminal 17 selects any of the multiple-phaseclocks generated by the multiple-phase output analog PLL 65, and thephase of the clock is shifted with respect to the clock synchronizedwith the horizontal sync signal, and then the clock is outputted fromthe clock output terminal 16. This clock is also used as a samplingclock of the ADC 43.

As described above, in the clock generation apparatus according to thefifth embodiment, a multiple-phase clocks are generated by using themultiple-phase output analog PLL 65, and any of the clocks is selectedby the clock selection circuit 64 to shift the phase of the clock.Therefore, as in the second embodiment, when sampling a personalcomputer signal or the like, a clearer image can be obtained, andfurther, only the sampling clock of the ADC 43, i.e., the clock forsampling the video signal, can be phase-shifted, whereby disturbance atphase change can be suppressed. Further, this circuit can be realized byreplacing the originally used analog PLL with the multiple-phase outputanalog PLL, thereby reducing the area of a semiconductor device or thelike when the circuit is incorporated in the semiconductor device.

Embodiment 6

FIG. 15 is a block diagram illustrating the construction of a clockgeneration apparatus according to a sixth embodiment of the presentinvention. In FIG. 15, the same reference numerals as those shown inFIG. 5 denote the same or corresponding parts, and reference numeral 90denotes a frequency spread information generation circuit, 91 denotes aDTO, 92 denotes a DAC, 93 denotes an analog PLL, and 94 denotes a clockoutput terminal.

Hereinafter, the operation of the clock generation apparatus accordingto the sixth embodiment will be described with reference to thedrawings. Initially, the operation of the circuit comprising the ADC 2,the horizontal sync signal separation circuit 6, the phase comparator 7,the digital LPF 8, the DTO 10, the DAC 11, the analog PLL 12, the MCKinput terminal 13, and the signal input terminal 40 is identical to theoperation of the corresponding part of the clock generation apparatusaccording to the second embodiment. The clock generation apparatusaccording to the sixth embodiment is provided with the frequency spreadinformation generation circuit 90 to generate frequency spreadinformation, and the second DTO 91 and the second DAC 92 generate asinusoidal wave on the basis of the phase error information and thefrequency spread information which are outputted from the digital LPF 8,and then the sinusoidal wave is multiplied, thereby outputting afrequency-spread clock from the clock output terminal 94.

FIG. 16 is a block diagram illustrating the construction of the DTO 91used in the clock generation apparatus according to the sixthembodiment. In FIG. 16, reference numeral 20 denotes a phase errorinformation input terminal, 21 denotes a center frequency informationinput terminal, 22 denotes an adder, 23 denotes a flop-flop, 24 denotesa SIN-ROM, and 25 denotes a waveform output terminal. These constituentsare identical to those of the DTO according to the first embodiment.Further, reference numeral 95 denotes a frequency spread informationinput terminal. Hereinafter, the operation of the DTO 91 will bedescribed. The operations of the phase error information input terminal20, the center frequency information input terminal 21, the adder 22,the flip-flop 23, the SIN-ROM 24, and the waveform output terminal 25are identical to those of the DTO 10 according to the first embodiment.The frequency spread information is inputted from the frequency spreadinformation input terminal 95, and the center frequency information andthe phase error information are added to the frequency spreadinformation by the adder 22. Further, by adding the frequency spreadinformation using the adder 22, the circuit can be easily digitized,thereby incorporation of the circuit into a semiconductor device or thelike can be facilitated.

FIG. 17 is a block diagram illustrating the construction of thefrequency spread information generation circuit 90 included in the clockgeneration apparatus according to the sixth embodiment. FIG. 18 is atiming waveform diagram of the frequency spread information generationcircuit 90. In FIG. 17, reference numeral 96 denotes a clock inputterminal, 97 denotes a timing generation circuit, 98 denotes an Up/Downcounter, and 99 denotes a frequency spread information output terminal.

Hereinafter, the operation of the frequency spread informationgeneration circuit 90 will be described. A clock is inputted through theclock input terminal 96, and a timing for making frequency spreadinformation is generated by the timing generation circuit 97. In thissixth embodiment, a clock whose frequency is divided (frequency dividedclock) is generated like an Up/Down switch signal N. The frequencyspread information P can be generated by performing addition andsubtraction with the Up/Down counter 98 according to the logic values“1” and “0” of the frequency divided clock. This waveform is outputtedfrom the frequency spread information output terminal 99, and thefrequency can be spread so that the frequency is heightened at therising edge of the frequency spread information P and lowered at thefalling edge thereof. Thus, the frequency spread information generationcircuit 90 is composed of only the frequency division circuit and theUp/Down counter to facilitate incorporation of the circuit into asemiconductor device or the like.

As described above, the clock generation apparatus according to thesixth embodiment is provided with the frequency spread informationgeneration circuit 90 to generate a clock including frequency spreadinformation, and the frequency spread clock is used for a system,whereby interference of the clock frequency which has conventionallyoccurred can be frequency spread, resulting in a reduction ininterference at a specific frequency. Further, the digital frequencyspread information generated by the timing generation circuit 97 and theUp/Down counter 98 is added as an input to the DTO 91, whereby controlcan be carried out with digital data, and addition of arbitrary spreadinformation is facilitated.

Embodiment 7

FIG. 19 is a block diagram illustrating the construction of a clockgeneration apparatus according to a seventh embodiment of the presentinvention. In FIG. 19, the same reference numerals as those shown inFIG. 15 denote the same or corresponding parts, and reference numeral101 denotes a PWM, 102 denotes an analog LPF, 103 denotes an analog PLL,and 104 denotes a clock output terminal.

Hereinafter, the operation of the clock generation apparatus accordingto the seventh embodiment will be described with reference to thedrawings. Initially, the operation of the circuit comprising the ADC 2,the horizontal sync signal separation circuit 6, the phase comparator 7,the digital LPF 8, the DTO 10, the DAC 11, the MCK input terminal 13,the signal input terminal 40, the clock output terminal 14, and thefrequency spread information generation circuit 90 is identical to theoperation of the corresponding part of the clock generation apparatusaccording to the sixth embodiment. In the clock generation apparatusaccording to the seventh embodiment, frequency spread informationgenerated in the frequency spread information generation circuit 90 isconverted into an analog value by the PWM 101 and the analog LPF 102.This analog value is added to the analog PLL 103 to generate a frequencyspread clock from the analog PLL 103, and the frequency spread clock isoutputted from the clock output terminal 104.

FIG. 20 is a block diagram illustrating the analog PLL 103 of the clockgeneration apparatus according to the seventh embodiment. In FIG. 20,reference numerals 70 to 74 denote inversion delay elements, 75 denotesa frequency division circuit, 76 denotes a phase comparator, 77 denotesan LPF, 78 denotes a bias circuit, and 79 denotes an REF signal inputterminal. These constituents are identical to those of themultiple-phase output analog PLL 65 according to the fifth embodiment.Further, reference numeral 110 denotes an analog frequency spreadinformation input terminal, 111 denotes a resistor, and 112 denotes aclock output terminal.

Hereinafter, the operation of the analog PLL 103 will be described. Theoperations of the inversion delay elements 70 to 74, the frequencydivision circuit 75, the phase comparator 76, the LPF 77, the biascircuit 78, and the REF signal input terminal 79 are identical to theoperations of those of the multiple-phase output analog PLL 65 accordingto the fifth embodiment.

In this seventh embodiment, analog frequency spread informationoutputted from the analog LPF 102 is received by the analog frequencyspread information input terminal 110, and added to the output of theLPF 77 through the resistor 111, whereby a voltage that controls theinversion delay elements 70 to 74 slightly varies, and a frequencyspread clock can be output from the clock output terminal 112.

As described above, since the clock generation apparatus according tothe seventh embodiment is provided with the frequency spread informationgeneration circuit 90, the PWM 101, and the analog LPF 102, it ispossible to generate analog value frequency spread information. Byapplying, to a system, a clock that is frequency spread using thisfrequency spread information, interference of clock frequency which hasconventionally occurred can also be frequency spread. Further, since thePWM 101 and the analog LPF 102 are provided to generate analog valuefrequency spread information, the second DAC in the second DTO used inthe clock generation apparatus according to the sixth embodiment can bedispensed with.

Embodiment 8

FIG. 21 is a block diagram illustrating the construction of aclock-generation apparatus according to an eighth embodiment of thepresent invention. In FIG. 21, the same reference numerals as thoseshown in FIG. 1 denote the same or corresponding parts, and referencenumeral 120 denotes a frequency phase calculation circuit, 121 denotes aDTO, 122 denotes a DAC, 123 denotes an analog PLL, and 124 denotes aclock output terminal.

Hereinafter, the operation of the clock generation apparatus accordingto the eighth embodiment will be described with reference to thedrawings. The operation of the circuit comprising the video signal inputterminal 1, the ADC 2, the burst extraction circuit 3, the phasecomparator 7, the digital LPF 8, the DTO 10, the DAC 11, the analog PLL12, the MCK input terminal 13, and the clock output terminal 14 isidentical to the operation of the corresponding part of the clockgeneration apparatus according to the first embodiment.

In the clock generation apparatus according to the eighth embodiment,frequency information (burst lock clock frequency information) of theDTO 10 and phase error information of a clock with respect to ahorizontal sync signal outputted from the digital LPF 8 are calculatedby the frequency phase calculation circuit 120 to output frequencyinformation and phase information which are needed to generate a clocksynchronized with the horizontal sync signal in the DTO 121, and asinusoidal wave is generated by the DTO 121 and the DAC 122. Thesinusoidal wave is multiplied by the analog PLL 123, whereby a desiredline lock clock can be outputted from the clock output terminal 124.Therefore, both of the burst lock clock and the line lock clock can besimultaneously generated with the stable digital PLL, using only oneADC, whereby a clock synchronized with the burst signal and a clocksynchronized with the horizontal sync signal can be simultaneouslygenerated without using a high multiplication analog PLL that issensitive to noise, even when a burst lock clock is needed for an inputsystem and a line lock clock is needed for a display system, like aliquid crystal television or the like.

FIG. 22 is a simple block diagram illustrating the frequency phasecalculation circuit 120 and the DTO 121 in the clock generationapparatus according to the eighth embodiment. Further, FIG. 23 is adetailed block diagram including the frequency phase calculation circuit120 and the DTO 121 including the DTO 10. FIG. 24 is a waveform diagramfor explaining the conception of a frequency calculation method. FIG. 25is a waveform diagram for explaining a phase information calculationmethod. In these figures, reference numeral 130 denotes a phase errorinformation input terminal, 131 denotes a burst lock clock frequencyinformation input terminal, 132 denotes a frequency informationcalculation circuit, 133 denotes an adder, 134 denotes a burst lockclock phase information input terminal, 135 denotes a phase informationcalculation circuit, 136 denotes a selector, 137 denotes a flip-flop,138 denotes a timing pulse input terminal, and 139 denotes an outputterminal.

Hereinafter, the operation of the frequency phase calculation circuit120 will be described with reference to the drawings. In this eighthembodiment, a description will be given of an NTSC system televisionsignal that is used in Japan and North America. It is assumed that aburst lock clock is 4 multiplication of a burst, a line lock clock is1280 multiplication of the horizontal sync signal, and each of theanalog PLL 12 and the analog PLL 123 performs 4 multiplication. Thefrequency of the DTO 121 comprising the adder 133, the selector 136, andthe flip-flop 137 depends on the frequency of the burst lock clock, thenumber of the burst lock clocks during one horizontal sync period, thecycle of the horizontal sync signal that is obtained on the basis of thephase error information within one clock, and the number ofmultiplication to be oscillated by the digital PLL. In this eighthembodiment, the frequency of the burst lock clock, the number of theburst lock clocks in one horizontal sync period, and the phase errorinformation within one clock are input to the frequency informationcalculation circuit 132. The cycle of the horizontal sync signal isobtained as follows.{cycle of horizontal sync signal=(number of burst lock clocks in onehorizontal sync period+phase error information within one clock)×cycleof burst lock clock}

Therefore, the cycle of the line lock clock is obtained by {cycle ofline lock clock=cycle of horizontal sync signal/number of multiplicationof the line lock clock}.

Accordingly, a clock of a desired frequency can be oscillated byperforming these calculations with the frequency information calculationcircuit 132 to obtain frequency information to the DTO 121. Further, thephase information calculation circuit 135 calculates phase informationthat is needed to perform phase matching required for line lock. Phasematching with the horizontal sync signal is carried out using the burstlock clock frequency information, the burst lock clock phaseinformation, the phase error information between the burst lock clockand the horizontal sync signal, and the line lock clock frequencyinformation that is calculated by the frequency information calculationcircuit 132. A phase error from a clock (mater clock) to be used by theDTO 10 that generates a burst lock clock can be calculated on the basisof the burst lock clock frequency information and the burst lock clockphase information. A phase error of the DTO 121 that generates a linelock clock is calculated using the calculated phase error, the line lockclock frequency information calculated by the frequency informationcalculation circuit 132, and the phase error information (output of thedigital LPF) between the burst lock clock and the horizontal syncsignal, and the selector 136 is controlled with a timing pulse that isoutputted one time during the horizontal period from the timing pulseinput circuit, and an initial value is inputted with the result obtainedby the phase information calculation circuit 135, thereby performingphase matching.

The operation of the frequency phase calculation circuit 120 accordingto the eighth embodiment will be further described with reference toFIGS. 23, 24, and 25. In FIG. 23, reference numeral 10 denotes a burstlock DTO, 121 denotes a line lock DTO, 130 denotes an input terminal forphase error information between the burst lock clock and the horizontalsync signal, 131 denotes a burst lock clock frequency information inputterminal, 132 denotes a frequency information calculation circuit, 135denotes a phase information calculation circuit, 141 and 142 denoteadders, 143 denotes a flip-flop, 144 denotes a burst lockfrequency-division ratio input terminal, 145 denotes a line lockfrequency-division ratio input terminal, 146 denotes a line lock/burstlock frequency ratio calculation circuit, 149 denotes a multiplier, 151denotes a horizontal sync signal input terminal, 152 denotes a timinggeneration circuit, 153 denotes a flip-flop, 155 denotes a phase errorcalculation circuit, 156 denotes an initial value calculation circuit,157 denotes an adder, 158 denotes a switch, 159 denotes a flip-flop, 160denotes a burst lock sawtooth wave output terminal, 161 denotes a linelock sawtooth wave output terminal, and 162 denotes line lock clockfrequency information. In FIG. 24, Q shows a MCK clock, R shows asawtooth wave outputted from the adder of the DTO 10, S shows a 4 fsclock that is obtained by multiplying the output of the DTO 10 with theanalog PLL 12, W shows a horizontal sync signal, T shows a sawtooth waveoutputted from the adder of the DTO 121, and U shows a line lock clockthat is obtained by multiplying the output of the DTO 121 with theanalog PLL 123. In FIG. 25, Q, R, S, W, and T are identical to thoseshown in FIG. 24, and V shows a sinusoidal wave outputted from the DTO10, and X shows a line lock clock before multiplication.

The frequency information calculation circuit 132 receives the number ofthe burst lock clocks during one horizontal sync period from the burstlock frequency-division ratio input terminal 144, the frequency of theburst lock clock from the burst lock frequency information inputterminal 131, the phase error information within one clock from thephase error information input terminal 130, and the frequency-divisionratio of the line lock clock to be output, from the line lockfrequency-division ratio input terminal 145. The number of the burstlock clocks during one horizontal sync period that is inputted from theburst lock frequency-division ratio input terminal 144 is obtained onthe basis of the horizontal sync signal extracted by the horizontal syncsignal separation circuit 6, and the center frequency information of theburst lock clock which is a fixed value. The frequency of the burst lockclock that is inputted from the burst lock frequency information inputterminal 131 is obtained by adding the output of the digital LPF 5 andthe center frequency information of the burst lock clock by the adder141. Since the cycle of the burst lock clock is identical to the cycleof the sawtooth wave outputted from the counter (flip-flop 143) of theDTO 10, the burst lock clock is shown as the sawtooth wave of the DTO 10in FIG. 24. As shown in FIG. 24, a factor k with which the frequency ofthe line lock clock is calculated by multiplying the burst lock clock byk is obtained as follows, using the number of sawtooth waves within onecycle of the horizontal sync signal, the phase error from the horizontalsync signal, and the frequency division ratio of the line lock clock tobe output.k={(number of burst lock clocks in one horizontal sync period+phaseerror information within one clock)/frequency division ratio of linelock clock to be output}

By calculating, with the multiplier 149, the factor k and the burst lockclock frequency information that is inputted from the burst lockfrequency information input terminal 131, line lock clock frequencyinformation 162 is obtained as an output of the multiplier 149. Theabove-mentioned construction enables frequency matching using onlycalculation, thereby providing a stable PLL. Further, the PLL can beconstructed by only the arithmetic elements of four arithmeticoperations, latch, and simple logic circuit, and therefore, the PLL caneasily be mounted on an integrated circuit.

Further, the operation of the phase information calculation circuit 135will be described with reference to FIG. 25. It is possible to obtain aphase difference between the burst lock clock and the MCK clock on thebasis of the phase error information inputted through the phase errorinformation input terminal 130, and the output value of the counter inthe DTO 10 at an edge (point a) of the MCK clock after a change point ofthe horizontal sync signal. The 0 point of the fsc clock (point b) issynchronized with the 0 point of the counter output of the DTO 10 and,therefore, these 0 points match. Accordingly, since gradient of thesawtooth wave of the counter of the DTO 10 is obtained from the burstlock clock frequency information, a phase difference between the burstlock clock and the MCK clock can be obtained as follows using the phaseerror information inputted through the information input terminal 130and the output value of the counter of the DTO 10 at point a:{phase difference between burst lock clock and MCK clock}={(output valuefrom counter of DTO 10 at point a/burst lock clock frequencyinformation)−phase error information inputted from information inputterminal 130}

Further, since the line lock clock frequency information 162 is obtainedby the frequency information calculation circuit 132, a value at point dof the counter output of the DTO 102, i.e., a value for initializing theDTO 121 can be obtained as follows.[value of counter output of DTO 102 at point d}=(line lock clockfrequency information 162×(phase difference between burst lock clock andMCK clock)}

Accordingly, in the phase information calculation circuit 135, a phasedifference between the MCK clock and the burst lock is calculated byinputting the phase error information between the burst lock clock andthe horizontal sync signal, which is supplied from the phase errorinformation terminal 130, to the phase error calculation circuit 155,inputting the burst lock clock frequency information supplied throughthe burst lock clock frequency information input terminal 131, inputtingthe horizontal sync signal through the horizontal sync signal inputterminal 151, extracting the rising edge in the timing generationcircuit 152, and inputting the phase information of the burst lock clockthat is outputted from the burst lock DTO 10 by the MCK immediatelyafter the rising edge. The output of the phase error calculation circuit155 and the line lock clock frequency information 162 are inputted tothe initial value calculation circuit 156 and calculated, whereby aninitial value for initializing the line lock DTO 121 can be calculated.Then, using the calculated value, the line lock DTO 121 is initializedat a timing synchronized with the horizontal sync signal, which isgenerated by the timing generation circuit 152, whereby the phase of theline lock clock generated by the line lock DTO 121 can be synchronizedwith the phase of the horizontal sync signal. Thereby, phase matchingcan be achieved by only arithmetic operations, resulting in a stablePLL. The circuit used in this eighth embodiment can be constituted bythe arithmetic elements of four arithmetic operations, latch, and simplelogic circuit, and therefore, it can be easily mounted on an integratedcircuit.

As described above, the clock generation apparatus according to theeighth embodiment is provided with a circuit for generating a clocksynchronized with the burst signal, which comprises the burst extractioncircuit 3, the first phase comparator 4, the first digital LPF 5, thefirst DTO 10, the DAC 11, and the first PLL 12, and a circuit forgenerating a clock synchronized with the horizontal sync signal, whichcomprises the horizontal sync signal separation circuit 6, the secondphase comparator 7, the second digital LPF 8, the frequency phasecalculation circuit 120, the second DTO 121, the second DAC 122, and thesecond PLL 123. Therefore, it is possible to realize a clock generationapparatus which can simultaneously generate a clock synchronized withthe burst signal and a clock synchronized with the horizontal syncsignal, which are required in a system such as a liquid crystaltelevision or the like.

Further, in the clock generation apparatus according to the eighthembodiment, the frequency phase calculation circuit 120 is provided withthe line lock clock frequency information generation circuit 132 whichreceives the phase error information of the burst lock clock as anoutput of the second digital LPF8, and the frequency information of theburst lock clock of the first DTO 10; and the line lock clock phaseinformation generation circuit 135 which receives the frequencyinformation of the line lock clock as an output of the line lock clockfrequency information generation circuit 132, the phase information ofthe burst lock clock of the first DTO 10, the frequency information ofthe burst lock clock, and the phase error information of the burst lockclock. Therefore, the frequency information and phase informationrequired for the line lock can be calculated using only digital data,whereby a stable system can be set up.

Further, in the clock generation apparatus according to the eighthembodiment, the line lock clock frequency information generation circuit132 is provided with the line lock/burst lock frequency ratiocalculation circuit 146 which receives the frequency division ratio ofthe burst lock clock to the horizontal sync signal during the standardoperation, the frequency division ratio of the line lock clock to thehorizontal sync signal during the standard operation, and the phasedifference between the signal obtained by frequency dividing the clockduring the burst lock clock standard operation and the actual horizontalsync signal, and calculates the ratio of frequencies between the signalobtained by frequency dividing the clock and the actual horizontal syncsignal; and the multiplier 149 which multiplies the frequencyinformation of the burst lock by the frequency ratio that is calculatedby the line lock/burst lock frequency ratio calculation circuit 146 tooutput the frequency information of the line lock. Therefore, thefrequency information can be generated by the ratio operation and theoperation of multiplying the factor obtained by the ratio operation,whereby the clock generation apparatus can easily be incorporated in adigital circuit.

Furthermore, in the clock generation apparatus according to the eighthembodiment, the line lock phase information generation circuit 135 isprovided with the timing generation circuit 152 which generates a timingto set the initial value of the second DTO that generates a line lockclock and a timing to capture such as the phase information of the burstlock clock; the line lock/burst lock frequency ratio calculation circuit155 which calculates the ratio of frequencies from the burst lockfrequency information and the line lock frequency information; and theinitial value calculation circuit 156 which calculates the initial valueof the second DTO that generates a line lock clock on the basis of thephase difference between the signal obtained by frequency dividing theclock during the burst lock clock standard operation and the actualhorizontal sync signal, and the frequency ratio obtained by the linelock/burst lock frequency ratio calculation circuit. Therefore, thecycle information of the DTO and the phase information of the DTO forthe burst lock and line lock can be obtained by four arithmeticoperations, whereby the clock generation apparatus can easily beincorporated in a digital circuit.

Embodiment 9

FIG. 26 is a block diagram illustrating the construction of a clockgeneration apparatus according to a ninth embodiment of the presentinvention. In FIG. 26, the same reference numerals as those shown inFIGS. 15 and 21 denote the same or corresponding parts.

Hereinafter, the operation of the clock generation apparatus accordingto the ninth embodiment will be described with reference to thedrawings. The operation of the circuit comprising the video signal inputterminal 1, the ADC 2, the burst extraction circuit 3, the phasecomparator 4, the digital LPF 5, the horizontal sync signal separationcircuit 6, the phase comparator 7, the digital LPF 8, the DTO 10, theDAC 11, the analog PLL 12, the MCK input terminal 13, the clock outputterminal 14, the frequency phase calculation circuit 120, the DAC 122,the analog PLL 123, and the clock output terminal 124 is identical tothe operation of the corresponding part of the clock generationapparatus according to the eighth embodiment. Further, the constructionand operation of the frequency spread information generation circuit 90are identical to those of the frequency spread information generationcircuit 90 of the clock generation apparatus according to the sixthembodiment.

In this ninth embodiment, only the frequency of the line lock clock isspread by inputting the frequency spread information outputted from thefrequency spread information generation circuit 90 into the DTO 121, andthe frequency-spread clock is used in the system, whereby interferenceof clock frequency which has occurred conventionally can also befrequency spread, resulting in a reduction in interference at a specificfrequency, and further, a reduction in vertical-striped interferencethat is specific to line lock.

Embodiment 10

FIG. 27 is a block diagram illustrating the construction of a clockgeneration apparatus according to a tenth embodiment of the presentinvention. In FIG. 27, the same reference numerals as those shown inFIGS. 19 and 21 denote the same or corresponding parts.

Hereinafter, the operation of the clock generation apparatus accordingto the tenth embodiment will be described with reference to thedrawings. The operation of the circuit comprising the video signal inputterminal 1, the ADC 2, the burst extraction circuit 3, the phasecomparator 4, the digital LPF 5, the horizontal sync signal separationcircuit 6, the phase comparator 7, the digital LPF 8, the DTO 10, theDAC 11, the analog PLL 12, the MCK input terminal 13, the clock outputterminal 14, the frequency phase calculation circuit 120, the DAC 122,the analog PLL 123, and the clock output terminal 124 is identical tothe operation of the corresponding part of the clock generationapparatus according to the eighth embodiment. Further, the constructionsand operations of the frequency spread information generation circuit90, the PWM 101, and the analog LPF 102 are identical to those of thefrequency spread information generation circuit 90, the PWM 101, and theanalog LPF 102 of the clock generation apparatus according to theseventh embodiment.

In this tenth embodiment, the frequency spread information outputtedfrom the frequency spread information generation circuit 90 is convertedinto an analog value by the PWM 101 and the analog LPF 102 and theanalog value is input to the voltage control terminal of the analog PLL,whereby only the frequency of the line lock clock is spread and thefrequency spread clock is used in the system. Thereby, interference ofclock frequency which has occurred conventionally can also be frequencyspread, resulting in a reduction in interference at a specificfrequency, and further, a reduction in vertical-striped interferencethat is specific to line lock.

APPLICABILITY IN INDUSTRY

The clock generation apparatus according to the present invention isprovided with the first phase comparator for comparing the phase of theburst signal with the phase of the clock of the ADC and the first LPF,and the second phase comparator for comparing the phase of thehorizontal sync signal with the phase of the clock and the second LPF,and either the first phase comparator or the second phase comparator isselected, whereby it becomes possible to generate clocks synchronizedwith various kinds of video signals, and therefore, the clock generationapparatus is useful for video signal processing of a television receiveror the like. Further, since the clock generation apparatus is providedwith the stationary phase error addition circuit for generating astationary phase error, the phase of a clock used for signal processingcan be shifted, and a stable part of data of the video signal can besampled by shifting the phase relationship between the clock to besampled with the ADC and the horizontal sync signal, whereby a clearimage can be obtained, and therefore, the clock generation apparatus isuseful in a video receiver such as a television receiver or a personalcomputer. Further, since the phase relationship between the clock to besampled by the ADC and the horizontal sync signal is shifted by changingthe phase of the clock to be outputted using the DLL and the multiplephase output PLL, a stable part of data of the video signal can besampled, whereby a clear image can be obtained, and therefore, the clockgeneration apparatus is useful in a video receiver such as a televisionreceiver or a personal computer. Furthermore, since the clock generationapparatus is provided with the frequency spread information generationcircuit, the frequency of the clock to be output can be consciouslyspread, whereby peaks of electromagnetic interference outputted from theclock can be reduced, and adverse effect of the clock to the video canbe reduced in a television receiver or a personal computer monitor videoreceiver. Moreover, since the second DTO is operated using the phaseerror information of the first phase comparator, the frequencyinformation of the first DTO, and the phase error information from thesecond phase comparator, it is possible to generate clocks synchronizedwith two signals such as a burst signal and a horizontal sync signal,respectively, using a single ADC, and therefore, the clock generationapparatus is useful in constituting a system that needs two clocks, suchas a liquid crystal television receiver in recent years. Further, theabove-mentioned constituents can be combined to realize the respectiveeffects at the same time, and therefore, are useful for enhancement inpicture quality of a television receiver or a monitor of a personalcomputer which deals with a video signal.

1. A clock generation apparatus comprising: an analog-to-digital converter (hereinafter referred to as an ADC) for converting a video signal on which a burst signal or a sync signal is superimposed, which is inputted through a video input terminal, into a digital signal, and outputting the digital signal; a burst signal extraction circuit for receiving the video signal digitized by the ADC, and extracting the burst signal from the video signal; a first phase comparator for comparing the phase of the burst signal extracted by the burst signal extraction circuit with the phase of a clock that operates the ADC; a first digital low-pass filter (hereinafter referred to as a digital LPF) for receiving the output of the first phase comparator, and smoothing the output of the first phase comparator to output phase error information; a horizontal sync signal separation circuit for receiving the video signal digitized by the ADC, and separating the horizontal sync signal from the video signal; a second phase comparator for comparing the phase of the horizontal sync signal that is separated by the horizontal sync signal separation circuit with the phase of the clock that operates the ADC; a second digital LPF for receiving the output of the second phase comparator, and smoothing the output of the second phase comparator to output phase error information; a switch circuit for receiving the output of the digital LPF and the output of the second digital LPF, and selecting one of these outputs; a digital timing oscillator (hereinafter referred to as a DTO) for receiving phase error information that is either the output of the first digital LPF or the output of the second digital LPF, which is outputted from the switch circuit, and outputting a digital timing signal which is generated at a timing according to the phase error information; a digital-to-analog converter (hereinafter referred to as a DAC) for converting the digital timing signal outputted from the DTO into an analog signal, and outputting the analog signal; and a PLL for supplying the ADC with a PLL clock according to the analog signal output from the DAC.
 2. A clock generation circuit comprising: an ADC for converting a video signal on which a sync signal is superimposed, which is inputted through a video input terminal, into a digital signal, and outputting the digital signal; a horizontal sync signal separation circuit for receiving the video signal digitized by the ADC, and separating the horizontal sync signal from the video signal; a phase comparator for comparing the phase of the horizontal sync signal that is separated by the horizontal sync signal separation circuit with the phase of a clock that operates the ADC; a digital LPF for receiving the output of the phase comparator, and smoothing the output of the phase comparator to output phase error information; a stationary phase error addition circuit for receiving the phase error information from the digital LPF, and adding stationary phase error to the phase error information; a DTO for receiving the phase error information outputted from the stationary phase error addition circuit, and outputting a digital timing signal that is generated at a timing according to the phase error information; a DAC for converting the digital timing signal outputted from the DTO into an analog signal, and outputting the analog signal; and a PLL for supplying the ADC with a PLL clock according to the analog signal output from the DAC.
 3. A clock generation circuit comprising: a first ADC for converting a video signal on which a sync signal is superimposed or a sync signal, which is inputted through a first video signal input terminal, into a digital signal, and outputting the digital signal; a second ADC for converting a video signal inputted through a second video signal input terminal into a digital signal, and outputting the digital signal; a horizontal sync signal separation circuit for receiving the video signal or sync signal that is digitized by the first ADC, and separating the horizontal sync signal from the signal; a phase comparator for comparing the phase of the horizontal sync signal that is separated by the horizontal sync signal separation circuit with the phase of a clock that operates the first ADC; a digital LPF for receiving the output of the phase comparator, and smoothing the output of the phase comparator to output phase error information; a stationary phase error addition circuit for receiving the phase error information from the digital LPF, and adding stationary phase error to the phase error information; a DTO for receiving the phase error information outputted from the stationary phase error addition circuit, and outputting a digital timing signal that is generated at a timing according to the phase error information; a DAC for converting the digital timing signal outputted from the DTO into an analog signal, and outputting the analog signal; and a PLL for supplying the first ADC and the second ADC with a PLL clock according to the analog signal output from the DAC.
 4. A clock generation circuit comprising: an ADC for converting a video signal on which a sync signal is superimposed, which is inputted through a video input terminal, into a digital signal, and outputting the digital signal; a horizontal sync signal separation circuit for receiving the video signal digitized by the ADC, and separating the horizontal sync signal from the video signal; a phase comparator for comparing the phase of the horizontal sync signal that is separated by the horizontal sync signal separation circuit with the phase of a clock that operates the ADC; a stationary phase error addition circuit for adding stationary phase error to the phase error information outputted from the phase comparator; a digital LPF for smoothing the output of the stationary phase error addition circuit; a DTO for receiving the phase error information outputted from the digital LPF, and outputting a digital timing signal that is generated at a timing according to the phase error information; a DAC for converting the digital timing signal outputted from the DTO into an analog signal, and outputting the analog signal; and a PLL for supplying the ADC with a PLL clock according to the analog signal output from the DAC.
 5. A clock generation circuit comprising: a first ADC for converting a video signal on which a sync signal is superimposed or a sync signal, which is inputted through a first video signal input terminal, into a digital signal, and outputting the digital signal; a second ADC for converting a video signal inputted through a second video signal input terminal into a digital signal, and outputting the digital signal; a horizontal sync signal separation circuit for receiving the video signal or sync signal that is digitized by the first ADC, and separating the horizontal sync signal from the signal; a phase comparator for comparing the phase of the horizontal sync signal that is separated by the horizontal sync signal separation circuit with the phase of a clock that operates the first ADC; a stationary phase error addition circuit for adding stationary phase error to the phase error information outputted from the phase comparator; a digital LPF for smoothing the output of the stationary phase error addition circuit; a DTO for receiving the phase error information outputted from the digital LPF, and outputting a digital timing signal that is generated at a timing according to the phase error information; a DAC for converting the digital timing signal outputted from the DTO into an analog signal, and outputting the analog signal; and a PLL for supplying the first ADC and the second ADC with a PLL clock according to the analog signal output from the DAC.
 6. A clock generation circuit comprising: a first ADC for converting a video signal on which a sync signal is superimposed or a sync signal, which is inputted through a first video signal input terminal, into a digital signal, and outputting the digital signal; a second ADC for converting a video signal inputted through a second video signal input terminal into a digital signal, and outputting the digital signal; a horizontal sync signal separation circuit for receiving the video signal or sync signal that is digitized by the first ADC, and separating the horizontal sync signal from the signal; a phase comparator for comparing the phase of the horizontal sync signal that is separated by the horizontal sync signal separation circuit with the phase of a clock that operates the first ADC; a digital LPF for receiving the output of the phase comparator, and smoothing the output of the phase comparator to output phase error information; a DTO for receiving the phase error information outputted from the digital LPF, and outputting a digital timing signal that is generated at a timing according to the phase error information; a DAC for converting the digital timing signal outputted from the DTO into an analog signal, and outputting the analog signal; a PLL for supplying the first ADC with a PLL clock according to the analog signal output from the DAC; a delayed lock loop (hereinafter referred to as a DLL) for receiving the clock outputted from the PLL, and delaying the clock to output multiple-phase clocks; and a clock selection circuit for selecting one of the multiple-phase clocks outputted from the DLL, and supplying the second ADC with the selected clock.
 7. A clock generation apparatus as defined in claim 6 wherein said DLL comprises a clock input terminal, N stages of variable delay elements for delaying a clock inputted through the clock input terminal, a phase comparator for comparing the outputs of the variable delay elements with a clock inputted through the clock input terminal, an LPF for smoothing the output of the phase comparator, a bias circuit for receiving the output of the LPF and controlling the delay values of the variable delay elements, and an output terminal for outputting the outputs of the N stages of variable delay elements.
 8. A clock generation circuit comprising: a first ADC for converting a video signal on which a sync signal is superimposed or a sync signal, which is inputted through a first video signal input terminal, into a digital signal, and outputting the digital signal; a second ADC for converting a video signal inputted through a second video signal input terminal into a digital signal, and outputting the digital signal; a horizontal sync signal separation circuit for receiving the video signal or sync signal that is digitized by the first ADC, and separating the horizontal sync signal from the signal; a phase comparator for comparing the phase of the horizontal sync signal that is separated by the horizontal sync signal separation circuit with the phase of a clock that operates the first ADC; a digital LPF for receiving the output of the phase comparator, and smoothing the output of the phase comparator to output phase error information; a DTO for receiving the phase error information outputted from the digital LPF, and outputting a digital timing signal that is generated at a timing according to the phase error information; a DAC for converting the digital timing signal outputted from the DTO into an analog signal, and outputting the analog signal; a multiple phase output PLL for multiplying the output of the DAC to supply a clock to the first ADC, and simultaneously, outputting an intermediate phase among the multiple phases; and a clock selection circuit for selecting one of the multiple phase clocks outputted from the multiple phase output PLL, and supplying the second ADC with the selected clock.
 9. A clock generation apparatus as defined in claim 8 wherein said multiple phase output PLL comprises a ring oscillator comprising M stages of variable delay elements, a frequency divider for converting the frequency of the output of the ring oscillator to 1/K, a phase comparator for comparing the phase of the output from the frequency divider with the phase of an REF signal inputted through an REF signal input terminal, an LPF for receiving the output of the phase comparator, a bias circuit for receiving the output of the LPF, and controlling the delay values of the variable delay elements, and an output terminal for outputting the outputs of the M stages of variable delay elements.
 10. A clock generation circuit comprising: an ADC for converting a video signal on which a sync signal is superimposed, which is inputted through a video input terminal, into a digital signal, and outputting the digital signal; a horizontal sync signal separation circuit for receiving the video signal digitized by the ADC, and separating the horizontal sync signal from the video signal; a phase comparator for comparing the phase of the horizontal sync signal that is separated by the horizontal sync signal separation-circuit with the phase of a clock that operates the ADC; a digital LPF for receiving the output of the phase comparator, and smoothing the output of the phase comparator to output phase error information; a first DTO for receiving the phase error information outputted from the digital LPF, and outputting a digital timing signal that is generated at a timing according to the phase error information; a DAC for converting the digital timing signal outputted from the first DTO into an analog signal, and outputting the analog signal; and a first PLL for supplying the first ADC with a PLL clock according to the analog signal output from the DAC; a frequency spread information generation circuit for generating frequency spread information; a second DTO for receiving the phase error information outputted from the digital LPF and the frequency spread information generated by the frequency spread information generation circuit, and outputting a digital timing signal; a second DAC for converting the digital timing signal outputted from the second DTO into an analog signal; and a PLL for outputting a PLL clock according to the analog signal output from the second DAC.
 11. A clock generation apparatus as defined in claim 10 wherein said frequency spread information generation circuit comprises a timing generation circuit which has a clock input terminal and counts the clocks inputted through the clock input terminal to generate a timing, and an up/down counter which receives a timing waveform generated by the timing generation circuit and switches between up counting and down counting.
 12. A clock generation circuit comprising: an ADC for converting a video signal on which a sync signal is superimposed, which is inputted through a video input terminal, into a digital signal, and outputting the digital signal; a horizontal sync signal separation circuit for receiving the video signal digitized by the ADC, and separating the horizontal sync signal from the video signal; a phase comparator for comparing the phase of the horizontal sync signal that is separated by the horizontal sync signal separation circuit with the phase of a clock that operates the ADC; a digital LPF for receiving the output of the phase comparator, and smoothing the output of the phase comparator to output phase error information; a DTO for receiving the phase error information outputted from the digital LPF, and outputting a digital timing signal that is generated at a timing according to the phase error information; a DAC for converting the digital timing signal outputted from the DTO into an analog signal, and outputting the analog signal; and a first PLL for supplying the first ADC with a PLL clock according to the analog signal output from the DAC; a second PLL for receiving the output of the DAC; a frequency spread information generation circuit for generating frequency spread information; a PWM circuit for converting the output of the frequency spread information generation circuit into an analog value; an analog LPF for receiving the output of the PWM circuit; and a resistor for adding the output of the analog LPF to a control voltage of a VCO which is a component of the second PLL.
 13. A clock generation apparatus as defined in claim 12 wherein said frequency spread information generation circuit comprises a timing generation circuit which has a clock input terminal and counts the clocks inputted through the clock input terminal to generate a timing, and an up/down counter which receives a timing waveform generated by the timing generation circuit and switches between up counting and down counting.
 14. A clock generation apparatus comprising: an ADC for converting a video signal on which a burst signal or a sync signal is superimposed, which is inputted through a video input terminal, into a digital signal, and outputting the digital signal; a burst signal extraction circuit for receiving the video signal digitized by the ADC, and extracting the burst signal from the video signal; a first phase comparator for comparing the phase of the burst signal extracted by the burst signal extraction circuit with the phase of a clock that operates the ADC; a first digital LPF for receiving the output of the first phase comparator, and smoothing the output of the first phase comparator to output phase error information; a first DTO for receiving the phase error information outputted from the first digital LPF, and outputting a digital timing signal that is generated at a timing according to the phase error information; a first DAC for converting the digital timing signal outputted from the first DTO into an analog signal, and outputting the analog signal; a first PLL for supplying the ADC with a PLL clock according to the analog signal output from the first DAC; a horizontal sync signal separation circuit for receiving the video signal digitized by the ADC, and separating a horizontal sync signal from the video signal; a second phase comparator for comparing the phase of the horizontal sync signal separated by the horizontal sync signal separation circuit with the phase of the clock that operates the ADC; a second digital LPF for receiving the output of the second phase comparator, and smoothing the output of the second phase comparator to output phase error information; a frequency phase calculation circuit for receiving frequency information and phase information of the output of the first DTO and, the phase error information outputted from the second digital LPF, and outputting frequency information and phase information which are needed to generate a clock synchronized with the horizontal sync signal; a second DTO for receiving the output of the frequency phase calculation circuit, and outputting a digital timing signal which is generated at a timing according to the output of the frequency phase calculation circuit; a second DAC for converting the digital timing signal outputted from the second DTO into an analog signal, and outputting the analog signal; and a second PLL for outputting a PLL clock according to an analog signal output from the second DAC.
 15. A clock generation apparatus as defined in claim 14 wherein said frequency phase calculation circuit comprises a line lock clock frequency information generation circuit for receiving phase error information between a burst lock clock and the horizontal sync signal, which is an output of the second digital LPF, and frequency information of a burst lock clock of the first DTO; and a line lock clock phase information generation circuit for receiving frequency information of a line lock clock outputted from the line lock clock frequency information generation circuit, phase information of the burst lock clock of the first DTO, the frequency information of the burst lock clock, and the phase error information between the burst lock clock and the horizontal sync signal.
 16. A clock generation apparatus as defined in claim 15 wherein said line lock clock frequency information generation circuit comprises a frequency ratio calculation circuit which receives the frequency division ratio of the burst lock clock to the horizontal sync signal during standard operation, the frequency division ratio of the line lock clock to the horizontal sync signal, and a phase difference between the actual horizontal sync signal and the signal obtained by frequency dividing the clock during burst lock clock standard operation, and calculates the ratio of frequencies between the burst lock clock and the line lock clock; and a multiplier which multiplies the frequency information of the burst lock clock by the ratio of the frequencies that is calculated by the frequency ratio calculation circuit to output frequency information of the line lock clock.
 17. A clock generation apparatus as defined in claim 15 wherein said line lock phase information generation circuit comprises a timing generation circuit for generating a timing for setting an initial value of the second DTO that generates the line lock clock, and a timing for capturing the phase information of the burst lock clock; a line lock/burst lock frequency ratio calculation circuit for calculating the ratio of frequencies from the burst lock frequency information and the line lock frequency information; and an initial value calculation circuit for calculating an initial value of the second DTO that generates the line lock clock from the phase difference between the actual horizontal sync signal and the signal obtained by frequency dividing the clock during burst lock clock standard operation, the phase information of the burst lock clock, and the frequency ratio obtained by the line lock/burst lock frequency ratio calculation circuit.
 18. A clock generation apparatus comprising: an ADC for converting a video signal on which a burst signal or a sync signal is superimposed, which is inputted through a video input terminal, into a digital signal, and outputting the digital signal; a burst signal extraction circuit for receiving the video signal digitized by the ADC, and extracting the burst signal from the video signal; a first phase comparator for comparing the phase of the burst signal extracted by the burst signal extraction circuit with the phase of a clock that operates the ADC; a first digital LPF for receiving the output of the first phase comparator, and smoothing the output of the first phase comparator to output phase error information; a first DTO for receiving the phase error information outputted from the first digital LPF, and outputting a digital timing signal that is generated at a timing according to the phase error information; a first DAC for converting the digital timing signal outputted from the first DTO into an analog signal, and outputting the analog signal; a first PLL for supplying the ADC with a PLL clock according to the analog signal output from the first DAC; a horizontal sync signal separation circuit for receiving the video signal digitized by the ADC, and separating a horizontal sync signal from the video signal; a second phase comparator for comparing the phase of the horizontal sync signal separated by the horizontal sync signal separation circuit with the phase of the clock that operates the ADC; a second digital LPF for receiving the output of the second phase comparator, and smoothing the output of the second phase comparator to output phase error information; a frequency spread information generation circuit for generating frequency spread information; a frequency phase calculation circuit for receiving frequency information and phase information of the output of the first DTO, and the phase error information outputted from the second digital LPF, and outputting frequency information and phase information which are needed to generate a clock synchronized with the horizontal sync signal; a second DTO for receiving the output of the frequency phase calculation circuit and the output of the frequency spread information generation circuit, and outputting a digital timing signal; a second DAC for converting the digital timing signal outputted from the second DTO into an analog signal, and outputting the analog signal; and a second PLL for outputting a PLL clock according to an analog signal output from the second DAC.
 19. A clock generation apparatus as defined in claim 18 wherein said frequency spread information generation circuit comprises a timing generation circuit which has a clock input terminal and counts the clocks inputted through the clock input terminal to generate a timing, and an up/down counter which receives a timing waveform generated by the timing generation circuit and switches between up counting and down counting.
 20. A clock generation apparatus comprising: an ADC for converting a video signal on which a burst signal or a sync signal is superimposed, which is inputted through a video input terminal, into a digital signal, and outputting the digital signal; a burst signal extraction circuit for receiving the video signal digitized by the ADC, and extracting the burst signal from the video signal; a first phase comparator for comparing the phase of the burst signal extracted by the burst signal extraction circuit with the phase of a clock that operates the ADC; a first digital LPF for receiving the output of the first phase comparator, and smoothing the output of the first phase comparator to output phase error information; a first DTO for receiving the phase error information outputted from the first digital LPF, and outputting a digital timing signal that is generated at a timing according to the phase error information; a first DAC for converting the digital timing signal outputted from the first DTO into an analog signal, and outputting the analog signal; a first PLL for supplying the ADC with a PLL clock according to the analog signal output from the first DAC; a horizontal sync signal separation circuit for receiving the video signal digitized by the ADC, and separating a horizontal sync signal from the video signal; a second phase comparator for comparing the phase of the horizontal sync signal separated by the horizontal sync signal separation circuit with the phase of the clock that operates the ADC; a second digital LPF for receiving the output of the second phase comparator, and smoothing the output of the second phase comparator to output phase error information; a frequency phase calculation circuit for receiving frequency information and phase information of the output of the first DTO, and the phase error information outputted from the second digital LPF, and outputting frequency information and phase information which are needed to generate a clock synchronized with the horizontal sync signal; a second DTO for receiving the output of the frequency phase calculation circuit, and outputting a digital timing signal that is generated at a timing according to the output of the frequency phase calculation circuit; a second DAC for converting the digital timing signal outputted from the second DTO into an analog signal, and outputting the analog signal; a second PLL for receiving the output of the second-DAC; a frequency spread information generation circuit for generating frequency spread information; a PWM circuit for converting the output of the frequency spread information generation circuit into an analog value; an analog LPF for receiving the output of the PWM circuit; and a resistor for adding the output of the analog LPF to a control voltage of the VCO which is a component of the second PLL.
 21. A clock generation apparatus as defined in claim 20 wherein said frequency spread information generation circuit comprises a timing generation circuit which has a clock input terminal and counts the clocks inputted through the clock input terminal to generate a timing, and an up/down counter which receives a timing waveform generated by the timing generation circuit and switches between up counting and down counting. 